Optical component and a method of fabricating an optical component

ABSTRACT

The invention relates to a method of manufacturing an optical component ( 800 ). It also relates to a branching unit and to a method of reducing insertion loss in an optical branching unit. It further relates to a method of reducing stress induced polarization effects in spaced planar waveguides (e.g. couplers) and stress induced tilting of the cores due to strain fields introduced by the top-cladding. It also relates to a method for filling high-aspect-ratio structures with material during reflow. The present invention proposes the use of additional structural elements such as transversal elements ( 850 ) connected to or pads ( 840, 841 ) or elongate elements located in the vicinity of ordinary waveguide core sections ( 801, 802 ). The additional structural elements are typically formed in the same processing step as the ordinary waveguide core sections. The additional structural elements have the purpose of enabling a better filling of small volumes between closely spaced waveguide core sections with cladding material thereby avoiding the creation of voids and to reduce the stress induced in neighbouring waveguide core sections thereby reducing birefringence. In some cases this or these effects may be combined with an improved control of the coupling of light from one waveguide to the other (such as in a directional coupler) or the provision of a gradual change in refractive index over a certain length between neighbouring waveguides (such as in a splitter). The invention may be used in connection with the distribution of signals in optical systems (e.g. CATV) or components, e.g. in the form of splitters and VOAs, as individual components or integrated on a chip.

This is a nationalization of PCT/DK03/00367 filed 4 Jun. 2003 andpublished in English, claiming benefit of U.S. provisional applicationNo. 60/385,891, filed Jun. 6, 2002 and Denmark application number PA200200855, filed Jun. 4, 2002.

TECHNICAL FIELD

The present invention relates to the field of planar optical componentsfor use in optical communications systems.

The invention relates specifically to a method of manufacturing anoptical component.

It also relates to a branching unit and to a method of reducinginsertion loss in an optical branching unit.

It further relates to a method of reducing stress induced polarizationeffects in spaced planar waveguides (e.g. couplers) and stress inducedtilting of the cores due to strain fields introduced by thetop-cladding.

It also relates to a method for filling high-aspect-ratio structureswith material during reflow.

The invention may e.g. be useful in applications such as opticalcommunications systems, specifically in connection with the distributionof signals in optical systems (e.g. CATV) or components, e.g. in theform of low loss, high uniformity splitters, couplers and variableoptical attenuators (VOAs), either as individual components orintegrated with other functions on a chip.

BACKGROUND ART

The following account of the prior art relates to one of the areas ofapplication of the present invention, optical communications systems.

Loss Reduction:

A planar optical branching component, such as a power Y-splitter, oftenplays a fundamental role in simple as well as complicated designs. Thefunction that a Y-splitter performs is to divide the incoming signalinto typically two signals of equal magnitude (50%:50%). It should benoted, that even though we in this text focus on the so-calledY-splitters, the presented idea can be used in branching components thatdivide the incoming signal(s) into more than just two output waveguidesas well as into varying splitting ratios, not just 50%:50%.

Often it is of interest to be able to divide one or more signals into alarge number of signals. To achieve this goal typically a so-calledsplitter tree is used where a number of fundamental splitters areconcatenated. A 1-4 splitter (tree) is then realized by concatenatingtwo 1-2 splitters to the two outputs of a first 1-2 splitter.Furthermore, a 1-8 splitter is realized by concatenating four 1-2splitters to the 1-4 splitter (one 1-2 splitter to each of the fouroutputs). In this way it is possible to realize 1-2^(N) splitter-treesusing only the simple 1-2 splitter. 2-2^(N) can be realized using asimilar approach where a 2-2 branching component (e.g. a coupler) isfollowed by two separate 1-2^(N−1) splitter trees.

Typically it is desirable that the splitter distributes the incomingsignal equally between the output waveguides and with minimum opticalloss. To achieve the goal of equal splitting, the individual splittersneed to divide the incoming signals equally, i.e. x %:x % where x is asclose as possible to 50.

If a splitter divides the signal equally and without loss, i.e. x=50,the intensity of the signal in the two output waveguides will beP_(output)=−10*log(½)=3.01 dB lower than the original signal. For aloss-less 1-4 splitter the power in the four output waveguides will beP_(output)=2*3.01=6.02 dB lower than the original signal, and for a 1-16splitter the output power will be P_(output)=4*3.01=12.04 dB than theoriginal signal. Thus for a 1-16 splitter the theoretical minimumreduction in the power level in an equally splitting component will be12.04 dB.

There is no such thing as a loss-less passive optical components sincethere will always be coupling losses, propagation losses, radiationlosses etc.

These loss factors can be minimized by proper choice of waveguidecross-sectional geometry, refractive index of the core material andcladding materials, as well as by choosing suitable curve-forms for thecomponent layout. By suitable choices we managed to fabricate low-loss1-16 components having typical values of insertion loss in theneighbourhood of 13.5 dB which is 13.5−12.04=1.46 dB above thetheoretical minimum. Assuming that the extra loss is equally distributedamong the four splittings in the 1-16 splitter tree the loss perfundamental splitter is 1.46 dB/4=0.365 dB (neglecting the coupling lossfrom the fibre to the chip). In order to come near the theoreticalminimum we thus need to reduce the excess loss per splitter by a fewtenths of a dB.

U.S. Pat. No. 5,745,618 discloses a 1-P power splitter comprising aninput waveguide and P output waveguides that are all coupled to a slabwaveguide in the form of a planar area, which is large compared to thearea of an individual waveguide and designed to support light wavetransmission between the input and output waveguides. The power splitterfurther comprises a transition region immediately adjacent to the slabwaveguide which comprises a number of silica paths (e.g. 30) thattransversely intersect the output waveguides. The silica paths aregenerally parallel to each other and have widths that progressivelydecrease as they become further away from the slab waveguide.

The present application discloses an optical branching component and amethod of reducing the loss of optical power in branching components, inparticular 1-2^(N) splitters, which makes possible the fabrication ofe.g. 1-16 (N=4) splitters having an insertion loss of approximately12.3-12.5 dB (typically).

Stress Relieving:

The present invention further deals with the issue of reducing stressinduced polarization effects and stress induced tilting of the cores inplanar optical components, e.g. waveguide couplers. Since thetop-cladding introduces a non-symmetric strain field across closelyspaced waveguides, measures have to be taken to minimize these effects.

One way to reduce this non-symmetric behaviour is to place the waveguidecores of the coupler as far apart as possible to approach an isolatedwaveguide situation. This way each waveguide core in the coupler willsee a quasi-symmetrical surrounding and the strain field will be moreuniform across the waveguide core. By placing the waveguide cores farapart in the coupler region, the length of the coupler device will besignificantly larger. Therefore this solution is not very suitable forcompact device design.

Other means of reducing this effect is by using a polymer over-claddingthat is heat-treated at low temperatures (<˜300° C.). Because of therelatively low process temperature, the thermally induced stress effectswill be smaller and consequently the stress levels lower. Polymertop-cladding has an inherent reliability problem and is thereforegenerally not used in commercial products.

Gap-filling:

The present invention further deals with the issue of gap-filling, whichfor planar waveguide fabrication is a technological challenge. Sincemost of the deposition processes are of a planar type, special measureshave to be taken to fill high-aspect-ratio trenches (e.g. trenches thathave a height to width ratio larger than 2 where the height dimension istaken in a direction of growth of the planar process and the widthdimension is taken perpendicular thereto). This can be done by addinge.g. boron and phosphorus to the glass whereby the flowing temperaturecan be reduced to typical anneal temperatures used in planar waveguidefabrication. The reflow-properties of the glass is, however, verydependent on the structures that have to be covered, e.g. a directionalcoupler. A directional coupler may be used either as an individualcomponent in itself, or as a part in a larger functionality. Adirectional coupler consists of two separate waveguides which, over adistance known as the coupling length (L_(CR)), are closely spaced (cf.FIG. 21).

If the two cores pertaining to the two separate waveguides aresufficiently close, the exponentially decaying tail of the optical fieldin the first waveguide core may be able to reach into the core of thesecond waveguide core. Being in the second waveguide core, the fieldpertaining to the first waveguide core is creating a polarization of theatoms in the core medium which in turn generates a new optical field inthe second core. The greater the magnitude of the field pertaining towaveguide 1 in waveguide 2, the greater is the polarization and hencethe faster the transfer of energy, which translates to a shorter lengthfor full coupling. In this way the energy in the field in the firstwaveguide core can be gradually transferred to the second waveguidecore. As the field decays exponentially outside the core regions ofeither of the waveguides, it is necessary that the two waveguide coresare closely spaced if good coupling and hence a short coupling length isto be achieved. If the distance between the two waveguide coresincreases, the distance along the length in which a certain percentageof the energy from the field in the first waveguide is coupled to thefield in the second waveguide increases exponentially. The smaller thecomponents, the more components per unit area or wafer may beimplemented, which—for directional couplers—requires that the waveguidecores in the coupling region be closely spaced.

In a directional coupler where the two cores are closely spaced, thedistance edge-to-edge between the two cores is small (e.g. less than 5μm apart or even less than 1 μm apart), especially relative to thewaveguide height—i.e. there will be a large aspect-ratio (waveguideheight divided by the edge-to-edge distance).

In a deposit-etch-deposit planar technology, a layer of core material isfirstly deposited on a lower cladding layer, secondly the core layer ispatterned using standard photolithographic techniques and the pattern istransferred to the core layer by etching. The pattern created during theetch step is finally covered and protected by deposition of a layer ofmaterial typically having optical characteristics as the lower claddinglayer.

If one uses a deposition method that does not deposit conformably ontothe underlying structures, i.e. does not deposit as fast (typicallyslower) on vertical faces (in a direction of growth or deposition) as onhorizontal faces (i.e. parallel to a planar face of the substrate andperpendicular to a direction of growth of planar layers), problems arelikely to arise in areas having large aspect-ratios. During depositionthe area above the narrow opening will gradually close while leaving thevolume between the two waveguides partly empty (i.e. comprising voids,so-called ‘keyholes’ or ‘air-pockets’). The deposition rate at thehorizontal face at the bottom of the narrow opening is considerablylower than on horizontal faces outside the coupling region, as thematerial flow into the volume between the two waveguides is restrictedby a shadowing effect from the waveguides themselves. Furthermore, thedeposition of material grows laterally (i.e. extending from side toside) from the upper corners of the waveguides towards the central partof the coupling region which further increases the shadowing effect. Theresult is that a void or air pocket is created in the region between thetwo waveguides (cf. FIG. 22.b).

The magnitude of the exponentially decaying evanescent field tail ismore or less exponentially dependent upon the refractive indexdifference of the core material and the surrounding material. Typicallythe refractive index of the material that surrounds the core will have avalue only slightly smaller than that of the core, in order to create astructure that is matched to a standard optical fibre, e.g. a SMF-28fibre. For such a fibre the core refractive index typically is around1.450 @ 1.55 μm, whereas the cladding refractive index has a refractiveindex around 1.445 @ 1.55 μm, i.e. a refractive index difference ofaround 5·10⁻³. If a void is present between the two closely spacedwaveguide cores of the directional coupler this void will constitute anarea having a refractive index 1 (that of a vacuum), hence therefractive index difference will now be around 0.450. This willobviously make the optical coupling between the two waveguidesuncontrollable and non-reproducible and thus render the componentuseless.

To ease the filling of the narrow space between the two waveguides inthe coupling region, a multi-step process is typically applied. In sucha process a layer of cladding material is deposited followed by ahigh-temperature treatment where the entire structure is heated to abovethe glass-transition temperature of the cladding material, which makesthe cladding material soft. When the cladding material is soft it canflow and redistribute itself (reflow) thereby better fill the narrowspaces. This process is repeated a number of times making it possible tocover most encountered structures. However, sometimes structures/designshaving aspect-ratios that prevent perfect filling of the narrow spacesare seen.

One solution to the problem of gap filling is to increase the dopinglevel of boron and phosphorus. This will “soften” the cladding materialeven further and thereby promoting the gap filling. However, the higherdoping concentration makes the glass less reliable and more susceptibleto water. It is therefore necessary to use hermetical packaging whichincreases the cost of the components.

Another solution to the problem is to use other deposition processessuch as flame-hydrolysis deposition (FHD) or Low Pressure ChemicalVapour Deposition (LPCVD). Both these processes have betterstep-coverage properties than plasma enhanced chemical vapour deposition(PECVD), but other factors such as lack of scalability, flexibility,control and automatic fabrication, etc. speak against these methods.

DISCLOSURE OF INVENTION

The present invention proposes the use of additional structural elementssuch as transversal elements connected to or pads or elongate elementslocated in the vicinity of ordinary waveguide core sections. Theadditional structural elements are typically formed in the sameprocessing step as the ordinary waveguide core sections. The additionalstructural elements have the purpose of enabling (an experimentallyobserved fact of) a better filling of small volumes between closelyspaced waveguide core sections with cladding material thereby avoidingthe creation of voids and to reduce the stress induced in neighbouringwaveguide core sections thereby reducing birefringence. In some casesthis or these effects may be combined with an improved control of thecoupling of light from one waveguide to the other (such as in adirectional coupler) or the provision of a gradual change in refractiveindex over a certain length between neighbouring waveguides (such as ina splitter), thereby reducing insertion loss.

A Method of Manufacturing an Optical Component Comprising TransversalElements:

Objects of the invention are achieved by a method of manufacturing anoptical component comprising a combination of planar waveguides on asubstrate, the combination of waveguides comprising spaced, parallel,diverging or merging waveguide core sections forming a core regionlayout in a planar view, the method comprising the steps of

-   a) providing a substrate,-   b) forming a lower cladding layer on the substrate,-   c) forming a core layer on the lower cladding layer,-   d) providing a core mask comprising a core pattern corresponding to    the core region layout and a layout of transversal elements, the    transversal elements extending between at least two of said spaced,    parallel, diverging or merging waveguide core sections, thereby    fully or partially connecting them,-   e) forming core sections and transversal elements using said core    mask, a photolithographic and an etching process, and-   f) forming an upper cladding layer to cover the waveguide core    sections, the transversal elements and the lower cladding layer    wherein at least one of the steps b), c), f) is performed by plasma    enhanced chemical vapour deposition.

Advantages of the invention are that a combined effect of preventing orminimizing the generation of voids in the cladding layer around closelyspaced waveguide core sections and the precise control of the refractiveindex in the region between spaced waveguides, potentially leading tolower losses of the component. The use of plasma enhanced chemicalvapour deposition (PECVD) in the manufacturing process has theadvantages of a commercially available, proven technology from manyyears of service in the semiconductor industry. PECVD is a flexiblemethod providing good control of essential parameters such as thermalexpansion, refractive index and thickness of corresponding layers of thecomponent.

The term ‘spaced’ is in the present context taken to mean that thecoupling of optical power between a waveguide and its ‘spaced’ neighbouris larger than 1/1000 of the total optical power propagated by thewaveguide in question. In an embodiment of the invention, the shortestdistance between faces perpendicular to the substrate of substantiallyparallel waveguides is less than twice the height of the waveguide inquestion, such as less than the height, such as less than half theheight of the waveguide in question. In an embodiment of the invention,the shortest distance between faces perpendicular to the substrate ofparallel waveguides is less than 10 μm, such as less than 5 μm, such asless than 1 μm.

The term ‘diverging’ is in the present context taken to mean extendingin different directions from a common point, i.e. as embodied e.g. inthe input and output waveguides of a coupler comprising two waveguidesthat are parallel over a certain length and diverge from each other(over a certain length) at both ends (cf. e.g. FIG. 21).

The term ‘merging waveguide core sections’ is in the present contexttaken to mean that two waveguide core sections meet and join to one in afork or Y-type structure such as in a splitter (cf. e.g. FIG. 1). Theterm split may just as well be used.

In an embodiment of the invention, the substrate is a silicon substrate,and the core and cladding layers comprise silica glass. The term ‘silicaglass’ is in the present context taken to mean a SiO₂ based glassoptionally comprising dopant elements such as boron, phosphorus,aluminium, fluorine, germanium, nitrogen, erbium (or other rare earthelements), titanium, etc. e.g. aimed at modifying the optical properties(e.g. refractive index) and/or the thermal expansion properties of theresulting material.

In an embodiment of the invention, the amount of dopant elements in theglass matrix is larger than 0.1 weight %, e.g. larger than 5 weight %,such as larger than 10 weight %.

In an embodiment of the invention, the upper cladding layer has a lowerflow temperature than that of the core and the lower cladding layer. Inan embodiment of the invention, the flow temperature of the uppercladding layer is adapted so that the waveguide core sections do notflow during an annealing that flows the upper cladding layer (at theanneal temperature and time in question). In an embodiment of theinvention, the flow temperature (in ° C.) of the upper cladding layer isat least 10% lower than that of the core and the lower cladding layersuch as at least 20% lower such as at least 50% lower. In an embodimentof the invention, the flow temperature of the upper cladding layer is atleast 20° C. lower than that of the core and the lower cladding layersuch as at least 50° C. lower such as at least 100° C. lower.

In general, all elements that lower the flow temperature may be used asdopant elements in appropriate amounts.

In an embodiment of the invention, the upper cladding layer comprisesboron and/or phosphorus. In an embodiment of the invention the amountsof boron and phosphorus are in total larger than 3 weight % such as eachlarger than 1 weight %.

In an embodiment of the invention, the optical characteristics of theupper and lower cladding layers are similar. In an embodiment of theinvention, the refractive index difference between the lower and uppercladding layers are less than 0.1% such as less than 0.05%, such as lessthan 0.01%.

In an embodiment of the invention, the formation of all layers on thesubstrate are made by plasma enhanced chemical vapour deposition. Othertechniques for applying layers to a substrate having problems with gapfilling may of course be used.

In an embodiment of the invention, step f) comprises successivedeposition and annealing steps. The annealing process should be carriedout at corresponding values of temperature and time allowing astabilization of the glass structures. In an embodiment of theinvention, the anneal temperature is between 800 and 1200° C., such as1000° C.

In an embodiment of the invention, the waveguide core sections that arefully or partially connected by transversal elements form part of acoupler or a splitter.

In an embodiment of the invention, the waveguide core sections that arefully or partially connected by transversal elements run essentiallyparallel over a certain length of the waveguides (e.g. 500-1000 μm) ofthe waveguides.

In an embodiment of the invention, the waveguide core sections that arefully or partially connected by transversal elements essentially divergefrom each other over a certain length (e.g. 500-1000 μm) of thewaveguides.

In an embodiment of the invention, at least one of the transversalelements fully connects two waveguide core sections.

Loss Reduction in a Splitter:

A problem of the prior art is that no matter how gently one parts thetwo output waveguide arms in the Y-splitter there will always be aradiation-loss contribution. Furthermore, using a deposit-etch-deposittechnology to fabricate the splitter (such as e.g. the silica-on-silicontechnology) there will be etch—as well as filling—problems where the twooutput waveguides are very closely spaced which can give rise toincreased propagation loss as well as unequal splitting ratios, hence itis of interest to part the two output waveguides as fast as possibleconsidering losses.

One way to circumvent the problem in the splitting region (the narrowarea between the two output waveguides) is to have a graded refractiveindex profile, such that the refractive index of the region between thewaveguides where the two waveguides part is close to the core refractiveindex and gradually decreases towards the cladding refractive index. Oneway to achieve this is to use grey-tone photolithography. This, however,requires complicated and expensive masks.

The introduction of a slab waveguide between the input and outputwaveguides represents another way of dealing with the problem in thesplitting region. This, however, has the disadvantage of giving arelatively uneven distribution of power between the individual outputwaveguides in a given cross section perpendicular to their transmissiondirection, the outer waveguides containing less power than the centrallypositioned output waveguides. Further, the introduction of a slab regionincreases the risk of exciting higher order modes, with a resultinggreater loss.

In the present application, we describe an optical branching componentwith reduced insertion loss and a method of its manufacture which isimplemented directly in the design of the component and does not requirethe number and character of fabrication processing steps to be modifiedin any way.

It is an object of the present invention to provide an optical branchingunit with reduced insertion loss. It is another object of the inventionto provide an optical branching unit with improved uniformity in thepower distribution between the individual branches of the unit.

It is a further object of the invention to provide an alternative methodof reducing insertion loss and improving uniformity in powerdistribution in a branching element, which method is relatively simpleand may be easily integrated in the normal processing of the branchingelement in question, and which allows a customized distribution ofoptical power between its individual output waveguides. It is anotherobject of the invention to provide a method which allows the manufactureof an optical branching unit that is easily scalable.

The objects of the invention are achieved by the invention described inthe accompanying claims and as described in the following.

An optical branching unit according to the invention is formed on asubstrate, the optical branching unit comprising waveguides for guidinglight at a predetermined wavelength λ, the waveguides comprising a coreregion, the core region being embedded in a cladding, the waveguidescomprising an input waveguide with a core region of width w_(in) and atleast two output waveguides having core region widths w_(out,i), abranching part for connecting the input and output waveguide cores, asplitting region adjacent to the branching part, the width of thebranching part being equal to w_(in) at its joint with the inputwaveguide core, the width of the branching part gradually expanding toallow the output waveguide cores to be branched off and diverge fromeach other in the splitting region, wherein a multitude of M transversalwaveguide core elements each having a width w_(i) and being embedded insaid cladding are located in the splitting region forming paths with amutual centre to centre distance of s_(i), said transversal waveguidecore elements fully or partially connecting neighbouring outputwaveguide cores. In an embodiment of the invention, the width of thebranching part is substantially equal to w_(in) at its joint with theinput waveguide core and to the sum of the widths w_(out,i) at its jointwith the output waveguide cores, the width of the branching partgradually expanding from its joint with the input waveguide core.

In an embodiment of the invention, the optical branching unit furthercomprises a parallel region adjacent to the splitting region, and theoutput waveguide cores run substantially parallel in the parallelregion.

In an embodiment of the invention, the core region has a refractiveindex n_(core) which may vary along the length and/or width of thewaveguide. In an embodiment of the invention, the cladding has arefractive index n_(clad) which is typically lower than the refractiveindex n_(core) of the core region. In an embodiment of the invention,the transversal elements have refractive indices n_(trans,i) which mayvary along the length and/or width of the elements. In an embodiment ofthe invention, n_(clad), n_(core) and n_(trans,i) are substantiallyconstant over the area covered by the optical branching unit and fulfilthe relation n_(clad)<n_(trans,i)<n_(core).

In an embodiment of the invention, the branching part has a refractiveindex n_(branch) equal to that of the core region, n_(core).

It is an advantage of an optical branching unit according to the presentinvention that it may be manufactured by PECVD, providing a potentiallylow cost, high-volume, reproducible and reliable component.

In an embodiment of the invention, the cladding comprises lower andupper cladding layers, the core region of a waveguide being formed in alayer applied to the lower cladding layer supported by the substrate andthe upper cladding layer being applied to cover the core region and thelower cladding layer.

In an embodiment of the invention, the upper cladding layer comprisesboron and/or phosphorus doped silica glass deposited by plasma enhancedchemical vapour deposition as a succession of individually annealedlayers.

The widths of the waveguide cores w_(in), w_(out) are to be taken in atransversal cross section (i.e. a cross section perpendicular to theintended direction of light guidance of said waveguide cores at thelocation of width measurement) as the dimension of the core region ofthe waveguide in question in a direction parallel to a reference planedefined by the opposing, substantially planar, surfaces of thesubstrate.

The widths of the transversal waveguide core elements w_(i) are to betaken as the dimension of the core in a direction parallel to thereference plane and to the direction given by the intended direction oflight guidance of the input waveguide core at the joint with thebranching part.

The width of the branching part is to be taken as the dimension of thepart in a direction parallel to said reference plane and in a crosssection perpendicular to the intended direction of light guidance ofsaid branching part at its joint with the input waveguide.

In an embodiment, the widths of the waveguide cores, the transversalwaveguide core elements and the branching element in a given transversalcross section are substantially constant in a direction perpendicular tothe reference plane (i.e. in a direction of substrate growth), in otherwords, they have a rectangular (possibly quadratic) cross section. In anembodiment, the height of the waveguide cores, the transversal waveguidecore elements and the branching element in a given transversal crosssection are substantially equal and given by the thickness of the corelayer.

In the present context, the terms ‘substantially constant’ or‘substantially planar’ or ‘substantially equal’ are taken to mean,respectively, constant and planar and equal within processingtolerances.

In the present context, the term ‘said transversal waveguide coreelements fully or partially connecting neighbouring output waveguidecores’ is taken to mean that the transversal waveguide core elementsform, respectively, an uninterrupted and an interrupted path betweensaid neighbouring output waveguide cores.

In the present context, the term ‘running substantially parallel in theparallel region’ is taken to mean that the output waveguides at leastdiverge less than in the splitting region, and preferably run parallelas obtainable within the processing tolerances in question.

The centre to centre distance s_(i) is taken to mean the perpendiculardistance between transversal waveguide core element i and element i+1,element 1 being closest to the branching part and element M farthestaway. s_(i) may or may not be a constant over the path of element i.

In an embodiment, the opposing edges of neighbouring diverging (orconverging when seen from the parallel region towards the branchingpart) output waveguide cores meet at a point—when seen in a planeparallel to said reference plane—at the joint with the branching part,the opposing output core edges in question running substantiallyperpendicular (within processing tolerances) to said reference plane. Inother words, the neighbouring output waveguide cores form a fork orY-type structure, in the latter case, the branching part resembling thepoints or a switch of a railway.

When opposing edges of neighbouring diverging output waveguide coresmeet at the joint with the branching part in a fork or Y-type structure,it is ensured that a particularly simple solution yielding a homogeneousdistribution of power between the output waveguides is provided.

When said branching part comprises a tapered part joining the input andoutput waveguide cores, the width of the tapered part beingsubstantially equal to w_(in) at its joint with the input waveguide coreand to the sum of the widths w_(out,i) at its joint with the outputwaveguide cores, and an abutting region, the output waveguide coreregions being aligned with and extending from said tapered region andabutting each other in the abutting region, it is ensured that analternative and from a design point of view particularly simple solutionyielding a homogeneous distribution of power between the outputwaveguides is provided and reducing the transition/radiation loss (i.e.non-guided light) in the transition between the straight input waveguideand the curved part of the output waveguide. In an embodiment, thetapering of the tapered part is gradual and continuous (withinprocessing tolerances), yielding an adiabatic taper. In an embodiment,the width w_(out,i) of the output vary over its length, i.e. outputwaveguide i has an initial width w_(out,ini,i) at the joint with thebranching part and a final width w_(out,fin,i) in the opposite (output)end. In an embodiment, w_(out,ini,i)<w_(out,fin,i) and further preferredW_(out,ini,i) is half the input waveguide core width(w_(out,ini,i)=0,5*w_(in)). This has the advantage that the risk ofexciting higher order modes, with a resulting greater loss, in thebranching part and its transition to the output waveguides isdiminished.

In an embodiment, the refractive indices of the waveguide core andcladding are substantially constant (step index profile), i.e. constantwithin processing variations, and preferably n_(core)=1,4520 andn_(clad)=1,4450 at λ=1550 nm. Alternatively, both indices may have aspatial dependence. The refractive index of the waveguide core may e.g.have a radial dependence (graded index profile) in a cross sectionperpendicular to the direction of guidance of light of the wavelength λ.

In an embodiment, the widths w_(out,i) of the individual outputwaveguide cores are substantially equal providing an equal distributionof the power between the output waveguides (assuming that the design ofthe transversal waveguide core elements does not cause a redistributionof power). Alternatively, the widths may be different and customized tospecific output power ratios between the individual output waveguides.

In an embodiment, the M transversal waveguide core elements forneighbouring output waveguides are placed in the splitting region over alength L extending from the joint between the output waveguides inquestion and the branching part. Preferably L should be large enough tominimize the insertion loss but not larger than necessary since theremaining transversal waveguide elements will scatter light from thecores and do not contribute to confining the light to the core.

The transversal waveguide core elements are preferably placed in thesplitting region over a length L taken in the output direction of theoptical branching unit, L being the sum of the individual centre tocentre distances s_(i), i=1, 2, . . . , M of the transversal waveguidecore elements.

Alternatively, the transversal waveguide core elements may extend intothe parallel region of the branching element.

In the present context, ‘the output direction of the optical branchingunit’ is defined by the intended direction of light guidance of theinput waveguide core at the joint with the branching part or—ifdifferent therefrom—a tangent to the abutting edge of the one (if thenumber of output waveguides P is uneven) or two (if P is even) centrallylocated output waveguide cores in the abutting region).

In an embodiment, the number of transversal waveguide core elements arein the range between 1 and 40. In general, the larger the number oftransversal waveguide elements, the more continuous a variation of theeffective refractive index of the splitting region and the lower loss isachieved. The actual number is determined as a compromise between designcomplexity (including a view to practical processing possibilities) andthe achieved incremental decrease in loss.

In an embodiment, the width w_(i) of the transversal waveguide coreelements for a given element no. i is substantially constant over itspath length.

In an embodiment, the individual centre to centre distances s_(i), i=1,2, . . . , M of the transversal waveguide core elements aresubstantially constant (but not necessarily equal for different i) overthe path of the i'th element for all i=1 to M—1. In other words, thetransverse elements form parallel or concentric or equivalent paths.Possible deviations there from may e.g. be used to tune the powerdistribution between the output waveguides, based on a simulation of thelayout design in question.

In an embodiment, the width w_(i) of the transversal waveguide coreelements decreases with increasing i as the output waveguide coresdiverge. In an embodiment, the width w_(i) of the transversal waveguidecore elements decreases linearly with distance as seen from the abuttingregion. In other words w_(i) decreases with increasing i.

In an embodiment, the centre to centre distance s_(i) between the i'thand the (i+1)'th transversal waveguide core element increases withincreasing i as the output waveguide cores diverge.

In both cases, this has the effect of gradually adjusting the effectiverefractive index of the region between two output waveguide cores tothat of the cladding. Preferably, the width w_(i) as well as the centreto centre distance s_(i) are, respectively, gradually (i.e. in steps)decreased and increased.

The above mentioned effective refractive index is defined in thefollowing: Instead of considering the true waveguide structure with coreand cladding the light propagation can for most situations easily bedescribed with great accuracy as a plane wave propagating in ahomogeneous medium having a refractive index n_(eff), the so-calledeffective refractive index. This effective index stems from eigenvalueequations originating from Maxwell's equations. The effective index of abound mode is greater than the cladding refractive index, and lower thanthe core refractive index. The effective index is furthermore a functionof the waveguide core cross-sectional geometry. See e.g. H. Nishiharaet. al. “Optical Integrated Circuits”, McGraw-Hill (1989).

In an embodiment, the refractive indices n_(trans,i) of the transversalwaveguide core elements are all equal. In a further preferredembodiment, n_(trans,i) equals the refractive index of the outputwaveguide cores n_(core). In further preferred embodiments, therefractive indices n_(trans,i) of the transversal waveguide coreelements are individually customized using dedicated doping or UVexposure presuming that the transversal waveguide core elements inquestion are made of a material sensitive to UV-light (such as aGe-doped silica glass). The refractive index n_(trans,i) may preferablybe decreased from transversal element to element with increasing i, e.g.from an initial value for i=1 equal to n_(core) to an end value for i=Mof n_(clad). This may be made as an alternative or a supplement to adecreasing width and/or an increasing centre to centre distance of thetransversal waveguide core elements.

In an embodiment, the individual tuning of the refractive indicesn_(trans,i) of the transversal waveguide core elements is used to tunethe distribution of power between the different output waveguides.

In an embodiment, the paths of the output waveguides are symmetricalabout a line defined by the centre of the input waveguide at its jointwith the branching part.

When the transversal waveguide core elements run substantially paralleland perpendicular to the output direction of the optical branching unit,it is ensured that a particularly low return loss is obtained.

In an embodiment, at least one and preferably all of the transversalwaveguide core elements form an uninterrupted path between twoneighbouring output waveguide cores. In an embodiment, all of the Mtransversal waveguide core elements connect two neighbouring outputwaveguide cores (i.e. form an uninterrupted, continuous path). In anembodiment, at least one of the transversal waveguide core elements istapered, i.e. has a gradual change of width over a length of its path.This may be used to tune the distribution of power between neighbouringoutput waveguides, the actual form and dimension being determined by asimulation depending on the actual process and geometrical parameters.

In an embodiment, at least one and preferably all of the transversalwaveguide core elements partially connect two neighbouring outputwaveguide cores (i.e. form an interrupted, optionally tapered, pathbetween them).

In an embodiment, the optical branching unit has 1 input and 2 outputsyielding the function of a Y-splitter. In other preferred embodiments,the optical branching unit has 1 input and 3, 4, 8 or 16 outputs. Thebranching units with more than 2 outputs are implemented by abutting thenumber of output waveguides in question in the abutting region anddiverging them from each other in the splitting region. In this mannerany 1-P power splitter may be manufactured.

Alternatively, a 1-2^(N) power splitter may be implemented as a splittertree, by using each of the output waveguides of a 1-2 power splitter asan input to a branching element as described above, yielding a 1-4 powersplitter. This may be continued to provide a 1-2^(N) power splitter.

The two types of splitters may be combined, e.g. by using each of the Poutputs of a 1-P power splitter as inputs to a 1-2 splitter, again usingeach of the 2P outputs of the P 1-2 splitters as inputs to 2P 1-2splitters, etc. Thereby a 1-P*2^(N) splitter may be implemented(P=1,2,3, . . . and N=0,1,2, . . . ), e.g. a 1-12 splitter by combininga 1-3 splitter with 9 1-2 splitters in 2 levels (3 and 6).

In an embodiment, the 1-Q splitter is connected at its input with an Xto Y multiplexing component yielding the function of an X to Y*Qdistribution component, which may e.g. be used to distribute severaldifferent input wavelengths to a number of different outputs, e.g. toachieve improved redundancy in a WDM-system or multiplex differentwavelengths into the same splitter, e.g. multiplexing wavelengths fromthe L band and the C band. In an embodiment, X equals 2. In anembodiment, Y equals 2. In an embodiment, Q equals 2.

The distribution of optical power between the individual outputwaveguides for an actual layout design may be controlled by (in aniterative process) simulating the power distribution to find an intendedratio between the individual output waveguides, while varying parameterssuch as refractive indices, width of output waveguide cores, number,location, width and form (e.g. tapering) of the transversal waveguidecore elements, etc.

A method of reducing insertion loss in an optical branching unitaccording to the invention is furthermore provided by the presentinvention. It comprises the steps of

-   -   providing a substrate and materials system    -   deciding a branching configuration and coupling geometry    -   deciding a core geometry with a view to said coupling geometry    -   deciding a refractive index difference between core and cladding        regions    -   designing a branching unit layout    -   simulate the mode field distribution in an iterative process        yielding appropriate combinations of core dimensions and        refractive indices, thereby adapting the layout    -   determining the minimum bending radius of curvature of the        branching unit output arms using a numerical method, preferably        a Beam Propagation Method    -   determining the number M, width w_(i), location and mutual        distance s_(i) of the transversal waveguide core elements by        iteration by        -   selecting a number M of elements        -   selecting a length L over which the M elements are to be            distributed        -   select a width w₁ of element 1        -   select a width w_(M) of element M        -   select widths of elements 2 to M−1 to be between w₁ and            w_(M) so that w_(i) decreases with increasing i        -   distributing the elements over the length L thereby            selecting their mutual distance s_(i)        -   calculating the total insertion loss of the branching unit            by a numerical method, preferably a Beam Propagation Method        -   varying the number of elements, their width and mutual            distance in an iterative process, ending when a minimum in            total insertion loss have been found.

Thereby it is ensured that an optical branching component with a lowinsertion loss and improved uniformity of the power distribution betweenthe individual output waveguides is provided.

The optical branching component may advantageously be implemented in allplanar waveguide systems such as silica-on-silicon, LiNbO₃,ion-exchange, silicon-on-insulator, III-V-systems and like.

When the length L is selected to be limited between the location of thejoint between the output waveguide cores and the branching part and alocation in the splitting region where the distance between theneighbouring diverging output waveguide cores is approximately 2 to 4times the width w_(out) of the output waveguide cores, it is ensuredthat the insertion loss is minimized.

An Optical Component Comprising Stress Relieving Structures:

An object of the present invention is to reduce the internal stress ofthe core region of a waveguide in an optical component, thereby reducingbirefringence.

The objects of the invention are achieved by the invention described inthe accompanying claims and as described in the following.

An optical component comprising a combination of planar waveguides on asubstrate is provided, each waveguide comprising a core region patternsurrounded by lower and upper cladding layers, the core region patternbeing formed in a layer applied to the lower cladding layer supported bythe substrate and the upper cladding layer being applied to cover thecore region pattern and the lower cladding layer, the combination ofwaveguides comprising spaced, parallel, diverging or merging waveguidecore sections. The component comprises a stress relieving elementlocated in the vicinity of the spaced, parallel, diverging or mergingwaveguide core sections.

It is an advantage of an optical component according to the presentinvention that it may be manufactured by PECVD, providing a potentiallylow cost, high-volume, reproducible and reliable component.

The term ‘a stress relieving element’ is in the present context taken tomean a structural element aimed at relieving stresses in the core regionpatterns. In an embodiment of the invention, the coefficient of thermalexpansion of a stress relieving element is less than that of thecladding material e.g. less than 90% such as less than 80% such as lessthan 50%. In an embodiment of the invention, the stress relievingelement or elements is/are made of the same material as the core regionpatterns. In an embodiment of the invention, the stress relievingelement or elements are formed in the same structural layer as the coreregion patterns. In an embodiment of the invention, the stress relievingelement or elements are formed in the same process step as the coreregion patterns. In an embodiment of the invention, the stress relievingelement or elements is/are made of the same material and in the sameprocess step as the core region patterns.

The term ‘in the vicinity of’ is in the present context taken to meanbeing positioned as close as possible relative to without substantiallyinfluencing the optical properties of the waveguide or waveguides aroundit (by not introducing substantial losses (such as larger than 1%) inthe waveguide). In an embodiment, the distance between substantiallyparallel faces perpendicular to the substrate of waveguide and stressrelieving elements is smaller than 15 μm, such as smaller than 10 μm,such as smaller than 5 μm.

The width and height of a waveguide is in the present context taken in atransversal cross section of the waveguide core (i.e. in a cross sectionperpendicular to the intended direction of light guidance of saidwaveguide cores at the location of a width measurement), the width beinga dimension of the core region of the waveguide in question in adirection parallel to a reference plane defined by the opposing,substantially planar, surfaces of the substrate, the height being adimension of the core region of the waveguide in question in a directionperpendicular to the reference plane (in a direction of growth). Whencomparing widths of waveguides and stress relieving elements anddistances between them, it is anticipated that the width of a stressrelieving element and the distance between a waveguide and a stressrelieving element are taken in the same cross section and direction asthe width of the waveguide in question. The width of a ridge (e.g. awaveguide) is generally taken as the largest width-dimension in thecross section in question (e.g.—but not necessarily—at the bottom of theridge closest to the supporting layer). The width of a groove (e.g. thedistance between waveguide core sections or between stress relievingstructures or between waveguide core sections and stress relievingstructures) is generally taken as the smallest dimension in the crosssection in question (e.g. at the bottom of the ridge closest to thesupporting layer).

In an embodiment of the invention, the width of a waveguide core istaken as the dimension defined by the corresponding mask used forgenerating the structure in question in the processing step forming thephysical layout of the waveguide core and additional structures.

In an embodiment of the invention, the distance between opposite orneighbouring faces of the spaced, parallel, diverging or mergingwaveguide core sections—over a certain length—is less than the height ofthe waveguide core sections, such as less than half the height, such asless than 0.1 times the height of the waveguide core sections, the facesbeing substantially parallel to a direction of growth of the core layer.

In an embodiment of the invention, the distance between a waveguide anda stress relieving element is smaller than 15 μm, such as smaller than10 μm, such as smaller than 5 μm.

In an embodiment of the invention, a stress relieving element iselongate and has a width that is less than or equal to the width of thenearest waveguide.

In an embodiment of the invention, the optical component comprisesseveral parallel running stress relieving elements. This has theadvantage of improving the uniformity of the strain field. In anembodiment of the invention, the distance between neighbouring stressrelieving elements is less than 15 μm, such as less than 10 μm, such asless than 5 μm.

In an embodiment of the invention, a stress relieving element has widthdimensions that are larger than the nearest waveguide.

In an embodiment of the invention, a stress relieving element has a formthat substantially matches the space between two merging or divergingwaveguide core sections. The actual geometry decides whether a solutionwith pads or elongate elements yields the better uniformity of thestrain field which may be determined by simulation.

In an embodiment of the invention, the optical component comprises abranching element such as a coupler or a splitter.

In an embodiment of the invention, the optical component furthercomprises transversal elements formed in the waveguide core layer andconnecting spaced, parallel, diverging or merging waveguide coresections.

The present invention further provides a method of manufacturing anoptical component comprising a combination of planar waveguides on asubstrate, the method being suitable for minimizing the internal stressof a waveguide and comprising the steps of

-   a) providing a substrate,-   b) forming a lower cladding layer on the substrate,-   c) forming a core layer on the lower cladding layer,-   d) providing a core mask comprising a core region pattern    corresponding to the layout of the core regions of waveguides of the    component and a pattern of stress relieving elements in the vicinity    of spaced, parallel, diverging or merging waveguide core sections,-   e) forming core regions and stress relieving elements using the core    mask, a photolithographic and an etching process, and-   f) forming an upper cladding layer to cover the core region pattern,    the stress relieving elements and the lower cladding layer.

In an embodiment of the invention, the substrate is a silicon substrate,and the core and cladding layers comprise silica.

In an embodiment of the invention, the upper cladding layer has a lowerflow temperature than that of the core and the lower cladding layer. Inan embodiment of the invention, the flow temperature of the uppercladding layer is adapted so that the waveguide core sections do notflow during an annealing that flows the upper cladding layer (at theanneal temperature and time in question). In an embodiment of theinvention, the flow temperature (in ° C.) of the upper cladding layer isat least 10% lower than that of the core and the lower cladding layersuch as at least 20% lower such as at least 50% lower. In an embodimentof the invention, the flow temperature of the upper cladding layer is atleast 20° C. lower than that of the core and the lower cladding layersuch as at least 50° C. lower such as at least 100° C. lower.

In an embodiment of the invention, the upper cladding layer comprisesboron and/or phosphorus. In an embodiment of the invention the amountsof boron and phosphorus are in total larger than 3 weight % such as eachlarger than 1 weight %.

In an embodiment of the invention, the optical characteristics of theupper and lower cladding layers are similar. In an embodiment of theinvention, the refractive index difference between the lower and uppercladding layers are less than 0.1% such as less than 0.05%, such as lessthan 0.01%.

In an embodiment of the invention, the formation of layers on thesubstrate is made by plasma enhanced chemical vapour deposition.

In an embodiment of the invention, step f) comprises successivedeposition and annealing steps. The annealing process should be carriedout at corresponding values of temperature and time allowing astabilization of the glass structures. In an embodiment of theinvention, the anneal temperature is between 800 and 1200° C., such as1000° C.

An Optical Component Comprising Waveguides with Transversal Elementsand/or Segmented Waveguides Aimed at Gap-filling:

The idea and hence the components are based on observations of whichtypes of waveguide core structures that typically can—and which typescannot—be filled with upper cladding without the creation of voids.

It is observed, that typically long parallel closely spaced structures(precisely as in directional couplers) give rise to problems in thecladding deposition.

It is an object of the present invention to provide a scheme for fillingout volumes around closely spaced, e.g. elongate, structural featuressuch as parallel or merging ridges on a substantially planar supportingface (e.g. waveguide core features standing out on a cladding layer)without introducing air pockets in a deposition process.

The objects of the invention are achieved by the invention described inthe accompanying claims and as described in the following.

An optical component comprising a combination of planar waveguides on asubstrate is provided, each waveguide comprising a core region patternsurrounded by lower and upper cladding layers, the core region patternbeing formed in a layer applied to the lower cladding layer supported bythe substrate and the upper cladding layer being applied to cover thecore region pattern and the lower cladding layer, the combination ofwaveguides comprising a length of at least two spaced waveguide coresections wherein transversal elements are arranged between said spacedwaveguide core sections.

It is an advantage of an optical component according to the presentinvention that it may be manufactured by PECVD, providing a potentiallylow cost, high-volume, reproducible and reliable component.

In an embodiment of the invention, the at least two spaced waveguidecore sections are essentially parallel. In an embodiment of theinvention, the component is a coupler.

To break the long canal between such two parallel, closely spacedwaveguide cores, a number of cross-bars is introduced into the spacebetween the waveguide cores to ease the flow of cladding material duringhigh-temperature treatments. The cross-bars may have another beneficialeffect, namely that of stabilizing the waveguide structure against thestress which arises when the cladding material flows. Because of themovement of the cladding material (and also because of different thermalexpansion coefficients), the surrounding material (i.e. the lower andupper cladding and the substrate) will tend to exert a pull in thecores. This pull will be counteracted by the cross-bars, which mayreduce the birefringence in the coupler-structure.

In an embodiment of the invention, the transversal elements are made ofthe same material as the core region patterns. In an embodiment of theinvention, the transversal elements are formed in the same structurallayer as the core region patterns. In an embodiment of the invention,the transversal elements are formed in the same process step as the coreregion patterns. In an embodiment of the invention, the transversalelements are made of the same material and in the same process step asthe core region patterns.

In an embodiment of the invention, at least one of the transversalelements physically connects to each of the spaced waveguide coresections. In an embodiment of the invention, each of the transversalelements physically connects to each of the spaced waveguide coresections.

In an embodiment of the invention, the optical component comprises twospaced, substantially parallel waveguide sections wherein the crosssections of the two waveguide sections and connecting transversalelements when viewed in a planar cross section (perpendicular to adirection of growth or deposition of layers) are mirror symmetric aroundan axis midway between the centre axes of the two waveguide sections.

In an embodiment of the invention, the transversal waveguide coreelements of a waveguide section are angled compared to an intendeddirection of light transmission of the waveguide section to minimizeback-reflections.

In an embodiment of the invention, the spaced waveguide core sectionsare segmented, each comprising a number of waveguide core piecesseparated by a space filled with upper cladding material.

In an embodiment, a waveguide core section and a transversal elementconnect at an angle larger than 45° such as larger than 60° such aslarger than 80°, such as around 82°.

In an embodiment of the invention, the optical component comprises astress relieving element located in the vicinity of spaced, parallel,diverging or merging waveguide core sections.

It has been observed that the object of the invention may be achieved byan alternative solution as discussed in the following.

An optical component comprising a combination of planar waveguides on asubstrate is provided, each waveguide comprising a core region patternsurrounded by lower and upper cladding layers, the core region patternbeing formed in a layer applied to the lower cladding layer supported bythe substrate and the upper cladding layer being applied to cover thecore region pattern and the lower cladding layer, the combination ofwaveguides comprising spaced, parallel, diverging or merging waveguidecore sections wherein said spaced, parallel, diverging or mergingwaveguide sections comprise segmented sections comprising a number ofseparate waveguide core pieces.

The term ‘segmented’ is in the present context taken to meannon-continuous, i.e. a segmented waveguide comprises physicallyunconnected waveguide core pieces. A segmented waveguide sectioncomprises a number of waveguide core pieces separated by a space filledwith upper cladding material.

In an embodiment of the invention, the optical component comprises atleast two spaced waveguide core sections which are essentially parallelover a certain length. In an embodiment of the invention, the componentis a coupler.

In an embodiment of the invention, each waveguide core piece isquadrilateral. In an embodiment of the invention, each waveguide corepiece has four edges when viewed in a planar cross section, the edgesconstituting two and two parallel opposing edges, i.e. togetherconstitute a parallelogram.

In an embodiment of the invention the optical component comprises anoptical coupler comprising two waveguides having—over a certainlength—substantially parallel sections of waveguides that diverge fromeach other at both ends of the parallel sections.

In an embodiment of the invention, the cross sections of the twosubstantially parallel waveguide sections, when viewed in a planar crosssection, are mirror symmetric around an axis midway between the centreaxes of the two waveguide sections, i.e. each waveguide segment in thefirst waveguide has its corresponding segment in the second waveguidewhich is the mirror image of the waveguide segment in the firstwaveguide.

In an embodiment of the invention, the spacing between each waveguidesegment is identical for all segments. In an embodiment of theinvention, the spacing between segments is less than 2 μm, such as lessthan 1 μm, such as less than 0.5 μm.

In an embodiment of the invention comprising two spaced waveguidesections, the angle of a parallelogram defining a waveguide piece asdefined by an edge of one waveguide section facing the other waveguidesection and the first edge encountered by light propagated in theintended direction of light transmission is larger than 90°, expressedas 90°+α.

In an embodiment of the invention, the transversal waveguide elementsmeet the corresponding waveguide segments at an angle substantiallyequal to 90−α.

In an embodiment of the invention, the angle α is around 8°.

In an embodiment of the invention, the optical component comprisestransversal waveguide core elements between segmented waveguidesections.

In an embodiment of the invention, the transversal waveguide coreelements of a waveguide section are angled compared to an intendeddirection of light transmission of the waveguide section.

In an embodiment of the invention, the transversal waveguide elementsmeet the corresponding waveguide segments at an angle substantiallyequal to 90−α. In an embodiment of the invention, the angle α is around8°.

In an embodiment of the invention, the optical component comprises astress relieving element located in the vicinity of spaced, parallel,diverging or merging waveguide core sections.

It should be emphasized that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other stated features, integers,steps, components or groups thereof.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with anembodiment and with reference to the drawings in which:

FIG. 1 shows a schematic partial view of a splitter according to theinvention, wherein a number of transversal elements of varying width andposition are inserted between the two output waveguide arms,

FIG. 2 shows the wavelength dependence of the total loss reduction andthe loss reduction for four different splitter designs in a 1-16splitter tree, each having been optimized individually according to theinvention compared to corresponding designs without the transversalelement structure,

FIG. 3 shows measured insertion loss for vertically and horizontallypolarized light as well as the polarization dependent loss for a 1-16splitter tree according to the invention,

FIG. 4 shows an example of an actual layout of an Y splitter accordingto the invention,

FIGS. 5.a and 5.b show examples of 2-4 branching units according to theinvention,

FIG. 6 shows an example of a 1-16 splitter according to the invention,

FIG. 7 shows a schematic cross sectional view of a branching elementaccording to the invention formed on a substrate with 8 output corewaveguides embedded in a cladding,

FIG. 8 shows a schematic cross sectional view of a branching elementaccording to the invention comprising a lower cladding layer formed on asubstrate with 8 output core waveguides applied to the lower claddinglayer and covered by an upper cladding layer,

FIG. 9 shows a schematic partial view of a 1 to 2 splitter according tothe invention, the splitter comprising transversal segmented elementsand stress relieving structures,

FIG. 10 shows a schematic partial view of a 1 to 3 splitter according tothe invention, the splitter comprising transversal elements of varyingwidth and position between the output waveguide arms,

FIG. 11 shows a typical birefringence effect at the centre of thewaveguide cores for a directional coupler, the black rectanglesindicating the position of the cores and the dashed line where thebirefringence is calculated,

FIG. 12 shows a birefringence calculation for one and two stressrelieving structures next to the cores, the stress relieve structuresbeing 1 μm wide,

FIG. 13 shows a close-up of the core birefringence area from FIG. 12,the asymmetry in the birefringence being reduced by applying stressrelieving structures,

FIG. 14 shows an embodiment of the stress relieving structures whereinthe wide structures are the waveguides and the narrow structures are thestress relieving structures,

FIG. 15 shows another embodiment of the stress relieving structureswherein the upper part has pads close to the coupler structures,

FIG. 16 shows a birefringence calculation for coupler structures withpads next to the cores,

FIG. 17 shows a coupler according to the invention comprising stressrelieving pads and segmented transversal elements over the couplinglength,

FIG. 18 shows a coupler according to the invention comprising stressrelieving pads and transversal elements over the coupling length and inthe regions of the coupler where the waveguides diverge/merge,

FIG. 19 shows a 1 to 2 splitter according to the invention comprisingstress relieving pads and transversal elements in the splitting region,

FIG. 20 shows a flow chart for a method of manufacturing an opticalcomponent according to the invention,

FIG. 21 shows a sketch of a traditional directional coupler with twoseparate waveguides closely spaced over a length L_(CR) (the couplinglength),

FIG. 22 schematically shows two waveguides forming a directionalcoupler;

FIG. 22.a shows a situation where the two waveguides are perfectlycovered, and FIG. 22.b shows a situation where the two waveguide coresare too closely spaced and the structure cannot be perfectly filledresulting in void formation between the two cores,

FIG. 23 shows a segment coupler according to the invention (type A)having N angled cross-bars of the width W and the distance betweenneighbours of S positioned in the space between the two waveguide cores,

FIG. 24 shows a segment coupler according to the invention (type B)wherein each of the two closely spaced waveguide cores in the couplingregion are segmented,

FIG. 25 shows a segment coupler according to the invention (type C)combining the features of the two types A and B of FIGS. 23 and 24, and

FIG. 26 shows a 3 to 3 directional coupler comprising transversalelements over the coupling length and in the regions of the couplerwhere the waveguides diverge/merge.

The figures are schematic and simplified for clarity, and they just showdetails which are essential to the understanding of the invention, whileother details are left out. Throughout, the same reference numerals areused for identical or corresponding parts.

MODE(S) FOR CARRYING OUT THE INVENTION

Loss Reduction in a Splitter:

FIG. 1 shows a schematic partial view of a 1-2 or Y-splitter 1 accordingto the invention, wherein a number of transversal elements 501, 502,503, 504, 505, 506, 507, 508, 509, 510 of varying width and position areinserted between the two output waveguide arms 301, 302. The splittercomprises an input core 2, and two output cores 301, 302 connected by abranching part 4 having core refractive index and consisting of atapered section 41 which adiabatically adapts the width of the inputcore to that of the sum of the two sideways abutted output core widthsand an abutting section 42 in which the output cores abut each otherover a certain length before they diverge in path into the splittingregion 7. Alternatively, the two output waveguides may overlap in the(possibly tapered) branching part (cf. 4 in FIG. 5.b). Alternatively,the two output waveguides may overlap in the abutting section 42 and mayalso be tapered. The transversal elements, having core refractive index,are located in the splitting region 7, i.e. the area between the twooutput waveguides from the Y-splitter, where the waveguides diverge fromeach other (as in FIG. 1). Alternatively the locations of transversalelements may extend into the area between the output waveguides wherethey run in parallel (the parallel region, cf. 8 in FIGS. 5.a and 5.b).The signature 70 indicates that the splitting region 7 is continued overa larger distance than shown in FIG. 1.

The input waveguide core 2, the output waveguide cores 301, 302, thebranching part 4 and the transversal elements 501-510 all have identicalcore refractive index and are embedded in a cladding 6, all formed on asubstrate (cf. 10 in FIG. 7).

In the processing of the device, the transversal elements are definedsimultaneously with the waveguide structure. The j'th element has awidth 52 of w_(j) and the separation 54 to the following element iss_(j). The length 53 in the output direction 50 over which the elementsare placed is denoted by L.

The effect of these transversal elements is to give rise to an“average/effective refractive index”. Where in the beginning theelements are wide and closely spaced, the “average/effective refractiveindex” is close to the core refractive index. Seen towards the right—inthe output direction 50—the widths of the elements are reduced (cf. e.g.the widths 51 (w_(i)) of the i'th element and 52 (w_(j)) of the j'thelement) and the separation of the elements is increased (cf. e.g. theseparation 54 (s_(j)) between the j'th and the (j+1)'th element and 55(s_(j+1)) between the (j+1)'th and the (j+2)'th element). This in turngives rise to a lower “average/effective refractive index” thatconverges towards the cladding refractive index. After the length L (ascounted from the location of the first element), the transversal elementstructure is terminated, L being the sum of the individual separationdistances s_(i).

FIG. 2 shows the dependence of the wavelength λ of the total lossreduction 90 and the loss reduction 91 (cf. notation on the figure) forfour different splitter designs in a 1-16 splitter tree (cf. FIG. 6),each having been optimized individually according to the inventioncompared to corresponding designs without the transversal elementstructure.

In order to find the optimum distribution of the transversal elements,their widths and the length L it is necessary to use iterativecalculations. This iteration is done by in turn varying the individualparameters, and then calculate the insertion loss for the component.This has been done for a 1-16 splitter design, and is depicted in FIG.2. As appears from FIG. 2, the total loss reduction 90 varies almostlinearly with wavelength in the range between 0.5 dB (λ=1.60 μm) and 0.9dB (λ=1.26 μm). The loss reduction per splitter 91 correspondingly varybetween approximately 0.1 and 0.2 dB in the same wavelength rangebetween 1.26 and 1.60 μm.

FIG. 3 shows measured insertion loss for vertically 95 and horizontally96 polarized light as well as the polarization dependent loss 97 for a1-16 splitter tree according to the invention.

Each of the four splitter designs constituting a 1-16 splitter tree havebeen optimized individually over the wavelength range typically ofinterest. The loss reduction for the four new designs have beencalculated, compared to the old designs without the transversal elementstructure in the splitting region, as well as the total loss reduction.In FIG. 3 measured insertion losses for light polarized 0° and 90° withrespect to vertical, as well as the polarization dependent loss(PDL=abs(Loss₀°−Loss₉₀°)) are shown. As appears from FIG. 3, PDL 97varies between 0.05 and 0.23 dB for the 16 outputs. The insertion lossesfor light polarized 0° 95 vary between −12.1 and −12.5 dB, whereas theinsertion losses for light polarized 90° 96 vary between −12.23 and−12.46 dB for the 16 outputs.

FIG. 4 shows an example of an actual layout of a Y splitter 1 accordingto the invention. The input 1 is connected to the branching part4—comprising a tapered part 41 and an abutting part 42, the latterconsisting of the two abutted ends of the output waveguide cores 301,302. A number (more than 30) of transversal elements 5 are insertedbetween the output waveguide cores over a length of the splitting regionstarting from the right after the abutting part. In determining thelocation of the first transversal element following the abutting part inthe output direction, the flow properties of the materials in questionmust be considered including the possibility to fill the relativelysmall area (volume) between the abutting part and the first transversalelement with cladding material.

FIGS. 5.a and 5.b show examples of 2-4 branching units according to theinvention. The device consists of a X %:(100-X)% field coupler followedby two 1-2 splitters according to the invention. It is preferred that Xis independent of wavelength and preferably 50. This can be obtained byproper design of the directional coupler, see, e.g. J. D. Love et al.Electron. Lett. 30 (1994) 1853-1854 or A. Takagi et. al. Electron. Lett.26 (1990) 132-133.

FIGS. 5.a and 5.b show a 2-4 branching unit 101 comprising a fieldcoupler 210 having two inputs 211, 212 and two outputs that serve asinputs to two identical 1-2 splitters with inputs 21, 22 joined by abranching part 4 to outputs 301, 302 and 303, 304, respectively withtransversal waveguide core elements 5 connecting the respective outputwaveguide cores over a fraction of the length of the splitting region 7.The output waveguides run in parallel in the output direction 50(defined by the direction of light guidance of the input waveguide) overa certain length in the parallel region 8.

In the branching unit of FIG. 5.a, the 2 outputs 301, 302 of an 1-2splitter are sideways abutted in an abutting part 42 of the branchingpart 4 joining the input 21 and output 301, 302 waveguide cores. Atapered part 41 adiabatically adapts the combined width of the twooutput waveguide cores to the width of the input waveguide core.

FIG. 5.b show a corresponding 2-4 branching unit 101 having the samestructure as the branching unit in FIG. 5.a apart from the branchingpart 4 of the 1-2 splitters, which in FIG. 5.b consists of a simpleY-structure resembling the points of a train railway, i.e. having oneinput gradually splitting into 2 outputs, the inputs and outputs havingidentical ‘track-widths’).

FIG. 6 shows an example of a 1-16 splitter according to the invention.The branching unit is composed of 1-2 Y-splitters in a splitter tree.Each 1-2 splitter is provided with a number of transversal waveguidecore elements connecting the 2 output waveguide cores. The lossreduction for individual parts of the 1-16 splitter is shown in FIG. 2.

The splitter tree of the 1-16 branching unit 110 of FIG. 6 comprises aninput 201 and 16 outputs 301-316 (the reference numbers of the 14outputs between 301 and 316 are not indicated). The inputs and outputsare joined by a splitter tree comprising 1-2 splitters at 4 levels asrepresented by 1-2 splitters 111, 112, 113, 114, each 1-2 splittercomprising transversal output waveguide core elements inserted foroptimum insertion loss. The layout of each individual splitter may beoptimized with respect to a minimum return loss. Alternatively (oradditionally), the insertion loss of the whole layout may be minimizedin an iterative process by variation of the design parameters for thecomponent, including the number, location, path, width and separation ofthe transversal waveguide core elements of each 1-2 splitter.

FIG. 7 shows a schematic cross sectional view of a branching elementaccording to the invention formed on a substrate with 8 output corewaveguides embedded in a cladding. FIG. 7 shows a cross section of aplanar waveguide structure according to the invention. The cross sectionis taken perpendicular to the intended direction of light guidance ofthe waveguides. The waveguide structure comprises a substrate 10, e.g. asilicon wafer, defining a reference plane 11. The substrate carries anumber of waveguides for guiding light comprising a cladding layer 6 inwhich waveguide core elements 301-308 are embedded. The waveguide coreelements have a rectangular cross section (within processingtolerances), the width being taken in a direction parallel to thereference plane 11 as indicated by 3081 for waveguide core 308. Thethickness of the waveguide core is given by its dimension in a directionperpendicular to the reference plane 11 as indicated by 3082 forwaveguide core 308. The thickness is controlled by the thickness of thecore layer during processing. The 8 waveguide core cross sections ofFIG. 7 may illustrate the 4×2 outputs of 4 1-2 splitters in a 1-8branching unit, i.e. (301, 302) and (303, 304) and (305, 306) and (307,308) constituting output waveguide pairs as seen in a cross section ofthe parallel region. The opposing edges of two neighbouring output coresmay be represented by edges 3051 and 3061 of output waveguide core pair(305, 306).

FIG. 8 shows a schematic cross sectional view of an optical componentaccording to the invention. The component is essentially identical tothat of FIG. 7. In FIG. 8, however, the cladding layer 6 is detailed toshow a lower cladding layer 61 formed on a substrate 10 with 8 outputwaveguide core sections 301-308 applied to the lower cladding layer 61,the waveguide core sections and the parts of the lower cladding layernot being covered by the waveguide core pattern is covered by an uppercladding layer 62. Additionally the upper cladding layer 62 is shown tohave a corrugated surface 621 due to repeated deposition and annealingsteps leading to reflow of the upper cladding layer (the amplitude isexaggerated in the drawing). The upper cladding layer has a lower flowtemperature than that of the core and lower cladding layers, controlledby proper addition of boron, phosphorus and/or fluorine (or any otherdopants that reduces the flow temperature) to the upper cladding layer.The control of the cladding reflow properties is e.g. described in R. A.Levy, K. Nassau, “Reflow Mechanisms of Contact Vias in VLSI Processing”,J. Electrochem. Soc., Vol. 133, No. 7, p. 1417 (1986), which isincorporated herein by reference.

FIG. 9 shows a one to two splitter 1 comprising an input waveguide coresection 2 and output waveguide core sections 301, 302 separated by abranching part comprising a tapered branching element 41. The branchingelement 41 adapts the width of the input core to that of the sum of thetwo sideways abutted output core sections. Between the output coresections a number of transversal elements 501, 502, 503 are positioned.Some of the transversal elements 502, 503 are segmented, i.e. compriseindividual pieces of core material 555 separated by a space filled withupper cladding material. The transversal elements are parallel, havedecreasing width, increasing mutual distance and comprise an increasingnumber of segments with increasing distance from the splitting point(i.e. the point at the branching part where the two output waveguidesseparate away from each other). Stress relieving structural elements inthe form of pads 340, 341 enclose the outer edges of the waveguide coresections of the component. The distance 370 between the neighbouringedges of, respectively, the waveguide core sections (including thebranching part) and the stress relieving pads is around 10 μm.

FIG. 10 shows a schematic partial view of a one to three splitter 1according to the invention. The splitter 1 comprises an input waveguidecore section 2 and output waveguide core sections 301, 302, 303separated by a branching part 41, 42 comprising a tapered branchingelement 41 and an abutting section 42. The branching element 41gradually adapts the width of the input core to that of the sum of thethree sideways abutted output core sections. A number of transversalelements 501, 502, 503 are positioned. between the three output coresections 301, 302, 303, the width of the transversal elements decreasingand their mutual distance increasing in the direction of lightpropagation from input to output as indicated by the arrow 50.

EXAMPLE 1 Fabrication of Splitter According to the Invention

In the following a description of one method to fabricate a branchingdevice according to the invention will be presented.

First, the core geometry and refractive index difference between coreand cladding materials should be fixed. For applications intelecommunications it is advantageous that there is as little aspossible insertion loss between the input-/output fibres and thecomponent as well as back-reflections from the fibre-componentinterface. Low insertion loss and low back-reflection is achieved bymatching the integrated waveguide structure to the fibres in geometry aswell as in refractive indices of the core and cladding materials. Intelecommunication systems typically the Corning SMF-28 or equivalentfibres are used. Using a commercial numerical mode solver, such as thecommercially available Selene from C2V, Enschede, The Netherlands, asuitable combination of core dimensions and refractive indexes is found.For good polarization dependence the core cross-sectional shape istypically quadratic. Now, given the core cross-sectional shape andrefractive index difference the minimum bending radius (radius ofcurvature) of the splitter output arms can be estimated using e.g.semi-analytical expressions or better numerical BPM (Beam PropagationMethod) methods. The minimum radius of curvature should be large enoughthat only a negligible amount of light is radiated out of the waveguidebend, but not too large as this yields a large and space consumingcomponent.

It is advantageous, but in no way necessary, to have a tapering regionin front of the two parting waveguides as this can reducetransition-/radiation loss from the initial straight waveguide to thebending waveguides.

When the various parts that constitute the traditional splitter havebeen chosen to yield a low-loss splitter, the transversal waveguideelements in the splitting have to be determined.

One method to determine the placement of the transversal elements is tofirst choose the length of the transition region L. L should be largeenough that the loss is minimized but not larger than necessary sincethe remaining transversal waveguide elements will slightly scatter lightfrom the cores and do not contribute to confining the light to the core.As a rule of thumb L should be chosen such that the inner edge-to-edgedistance between the bending waveguides is approximately 2-4 times thewaveguide core width.

When L is determined the transversal waveguide elements can be placed ina multitude of ways, however, the width (in the horizontal direction)should preferably decrease. This could be done e.g. by choosing amaximum (start) width, a minimum (end) width and a number M of elements,and then let the width w_(i) and the mutual distance s_(i) be a functionof i, where i=1,2, . . . , M.

After the elements have been placed in the design, the total loss iscalculated using a numerical method e.g. using BPM, e.g. thecommercially available Prometheus from C2V, Enschede, The Netherlands.The widths, spacings and the number N are subsequently variediteratively and the loss is calculated. When a minimum is found theiterations are ended.

EXAMPLE 2 A Variable Optical Attenuator According to the Invention

A VOA is an important device in network management for levellingmultiple wavelength signals. It can be realised by combining two 1-2splitters in a Mach-Zehnder geometry. The phase difference between lightthat propagates in the two arms can, e.g., be controlled by thethermo-optic effect by means of a heater element on one of the arms.Thereby the output intensity can be controlled. The advantage of thepresent invention is that the un-attenuated signal has low loss.Moreover, because of the better uniformity of the splitting ratio, alarger dynamic attenuation range can be expected (more than 25 dBattenuation).

Stress Relieving Structures:

The stress field across a coupler region is shown in FIG. 11,represented by the birefringence in the centre of the core region.ΔN_(y), ΔN_(x) represents the change of refractive index N in,respectively, the width (y) and height (x) directions, of a waveguidecore section between the anneal temperature and room temperature. As canbe seen there is a noticeable asymmetry of the birefringence in thewaveguide cores (the position of the cores in a cross sectionperpendicular to the intended direction of light propagation of thewaveguides are indicated by the black rectangles 801, 802). Thisasymmetry is believed to be one of the causes for un-predictedpolarization issues with directional couplers. To make thisbirefringence more uniform, stress relieving structures are added on theoutside of the coupler waveguides.

FIG. 12 shows the effect of adding one and two structures (821, 822 and820, 823, respectively) of 1 μm width at each side of the 6 μm wide and6 μm high coupler waveguides 801, 802, the coupler waveguides beingspaced 6 μm apart (edge-to-edge). The edge-to-edge distance between awaveguide and its nearest stress relieving structure is 5 μm and betweentwo stress relieving structures likewise 5 μm. The refractive indices ofthe waveguide core and cladding should be suitable for sustaining anoptical mode, e.g. substantially constant (step index profile),preferably n_(core)=1,4520 and n_(clad)=1,4450 at λ=1550 nm.

FIG. 13 is a close-up of the birefringence in the coupler waveguides. Itis clearly seen, that adding the stress relieving structures makes thebirefringence more uniform. This is an indication of a more uniformstrain field across the waveguides.

The width of the stress relieving grooves between a waveguide coresection and a stress relieving structure must be chosen such that no orlittle light is coupled from the waveguides and into the relievingstructures. The stress relieving structures can also be designed aslarge pads that are positioned with an appropriate distance to thecouplers, e.g. around 10 μm.

An embodiment of a waveguide coupler according to the invention is shownin FIG. 14. Here the wide lines 801, 802 are the waveguide corestructures and the narrow lines 810, 811, 812, 813, 820, 821 822, 823are the stress relieving structures. The stress relieving structures arepositioned around the waveguides in the whole coupling region 830 (thecentral part where the waveguides are closest). This will give a moreuniform strain field. The stress relieving structures will additionallyhave the advantage that the so-called gap-filling will be easier. Thismeans that void formation in the process of depositing and reflowing thetop-cladding will be eliminated.

Another embodiment of a waveguide coupler 800 according to the inventionis shown in FIG. 15. Here the stress relieving structures are designedas large pads 840, 841, 842, 843. The pads essentially fill out the(otherwise) open space between the waveguide core pattern of thecoupler. The distance from a pad 840 to a core 801 is in this caseapproximately 10 μm.

FIG. 16 shows the birefringence for a coupler as depicted in FIG. 15with and without the stress relieving pads (cf. 840, 841, 842, 843 ofFIG. 15). Also here, it is clear that the birefringence asymmetry in thecores is reduced by the inclusion of the pads.

FIG. 17 shows a coupler 800 according to the invention comprising stressrelieving pads 840, 841 and segmented transversal elements 850 extendingbetween the waveguide core sections 801, 802 over the coupling length.Each of the transversal elements 850 comprise a number of individualcore segments 855 separated by a space filled with upper claddingmaterial. The distance from a pad 840 to a core 801 is in this caseapproximately 15 μm.

FIG. 18 shows a coupler 800 according to the invention comprising stressrelieving pads 840, 841 and transversal elements 850 connecting thewaveguide core sections 801, 802 of the coupler over the coupling lengthand in the regions of the coupler where the waveguides diverge/merge.

FIG. 19 shows a one to two splitter 1 according to the inventioncomprising stress relieving pads 340, 341, 342 surrounding the input 2and output 301, 302 waveguide sections and the intermediate branchingelement 41. The component further comprises transversal elements 5located between the output waveguide sections in the splitting region.The distance 370 between the waveguide core sections (branching elementand transversal elements) and the stress relieving pads is preferably inthe range between 5 and 20 μm.

An Optical Component Comprising Segmented Waveguides and/or Waveguideswith Transversal Elements Aimed at Gap-filling:

FIG. 21 shows a sketch of a traditional directional coupler 800 with twoseparate waveguides 801, 802 closely spaced over a length L_(CR) (thecoupling length) 830.

FIG. 22 schematically shows two waveguide core sections forming adirectional coupler in a cross section perpendicular to the direction oflight guidance of the waveguides. FIG. 22.a shows a situation where thetwo waveguide cores 801, 802 are perfectly covered by an upper claddinglayer 62, and FIG. 22.b shows a situation where the two waveguide coresare too closely spaced and the structure cannot be perfectly filledresulting in void formation 805 in the upper cladding layer 62 betweenthe two cores 801, 802.

FIG. 23 shows a coupler 800 according to the invention, termed type A.In the space between the two waveguide cores 801, 802, N angledcross-bars 850 having a width W and a distance between neighbours S, areinserted. The N cross-bars (where N can be any number from 1 and up,depending on the actual structure) are inserted symmetrically around thecentral part of the coupling region 805 (the coupling region having thelength of L_(CR)). To reduce back-reflections each of the cross-bars 850are angled as shown in the insert figure. The angle α between an anglededge of a cross-bar and a direction perpendicular to the direction oflight propagation may preferably be in the neighbourhood of 8 degrees.However, the component will function with −45<α<45 degrees, which willnot impact the effects of improved reflow and reduced birefringence.

The width W, the number N and the distance between neighbours S dependon the given geometry of the coupler. It is self-evident that theinsertion of these cross-bars affects the optical coupling between thetwo waveguide cores, and the length of the coupling region L_(CR) iscalculated numerically using suitable numerical simulation software. Itcan be beneficial for optical loss reduction to distribute thecross-bars in such a way, that they e.g. primarily are placed in regionsin the coupler structure where the optical intensity is predominantly inone of the waveguide cores and omit cross-bars in regions where theintensity is predominantly between the two cores. In this way thecross-bars are less likely to give rise to loss and back-reflections.The geometry of the embodiment depicted in FIG. 23 is as follows:

-   -   Edge-to-edge distance between waveguide core sections 801, 802        over the coupling length L_(CR) 805 is 4 μm.    -   Coupling length L_(CR) 805 is 1000 μm.    -   Core width is 6 μm. Core height is 6 μm.    -   The number of cross-bars (also termed transversal elements in        other parts of the present application) is 20.    -   The width of the cross-bars is 2 μm.    -   The angle α is 8°.    -   The centre-to-centre distance between cross-bars is 50 μm.

FIG. 24 shows a coupler 800 according to the invention, termed type B.In a segment coupler of type B each of the two closely spaced waveguidecores 801, 802 are segmented having N segments 840. Each segment 840 ofthe embodiment in FIG. 24 have 4 edges 841, 842, 843, 844 constituting aparallelogram. The segments have the length S and a clearance betweensegments of W. The facets in the segments are angled, to reduceback-reflections, such that the facets form the angle α with vertical(in other words two non-opposing sides 842, 843 together define an angle90+α). Also for this coupler structure the length S and mutualseparations W may be optimized depending upon the actual structure. Thedistribution of W's and S's along the coupling region must also in thiscase be calculated numerically.

For this type of coupler the considerations regarding the coupler termedType A applies with minor modifications.

Since there are N punctures (i.e. the spacing between adjacent segmentsof core material) where the cladding material can flow more or lessfreely the reflow is eased hence a reduced stress-induced birefringenceis likely to result (of course depending on the geometries, dimensions,materials and processing details).

FIG. 25 shows a coupler 800 according to the invention, termed type C.This coupler type is a combination of the two formerly described types Aand B.

In segment coupler of type C the two closely spaced waveguide cores 801,802 are punctured with N spaces between N−1 segments 840 (as type B).The N−1 separate waveguide core segments are connected by cross-bars 850as described for the coupler of type A. Each cross-bar 850 is angled,i.e. comprises two pieces (or legs) 851, 852 that together define theangle 90+α. The considerations regarding types A and B apply to type Cas well. However, it is to be expected that this structure is morestable since the segments 840 are connected to each other by cross-bars850. For this structure all the appearing variables need be calculatednumerically in a recursive loop until the optimum structure is achieved(e.g. the width of the punctures and of the cross-bars need not beidentical).

FIG. 26 shows a three to three directional coupler 800 comprisingtransversal elements 850 between neighbouring waveguide core sections801, 802, 803 over the coupling length and in the regions of the couplerwhere the waveguides diverge/merge.

In general N, W and S are determined from the simple coupler structureas well as the viscosity of the cladding material during reflow. Usingsimulation software as e.g. Prometheus/OlympIOs from C2V the modifiedcoupling length is easily calculated compared to the traditional/simplestructure, and it is then possible to design a component which can berealised without voids, and which exhibit improved (reduced)birefringence.

Fabrication Technology

A branching component according to the present invention can befabricated in a number of different planar technologies such as inpolymers, in Silicon-on-insulator (SOI), Lithiumniobate (LiNbO₃), III-V,as well as in silica-on-Silicon and others. In an embodiment of thepresent invention the silica-on-silicon planar technology is used asthis technology produces the most advanced and technically developedplanar waveguide components. Silica waveguides possess a number ofhighly attractive properties such as material compatibility (opticalfibres are made from the same material, silica), optimum couplingbetween fibre and waveguide component (refractive indices and indexdifferences are comparable), low absorption- and propagation losses, lowbirefringence, high stability and low cost. Furthermore, the technologyused to fabricate these silica waveguides is identical to the technologyused in fabricating integrated electrical circuits such as CPU's(Central Processing Units in computers) and e.g. RAM (Random AccessMemory), thus this technology has matured during the last more thanthirty years and is known to be capable of mass production.

FIG. 20 shows a flow chart for a method of manufacturing an opticalcomponent according to the invention. The method comprises the steps a)to f) as discussed in the following.

In an embodiment of the present invention a clean and bare Silicon wafer(used as substrate, step a) is firstly oxidized (step b) to provide anoptical isolation layer of silica sufficiently thick that the magnitudeof the evanescent field tail of the field pertaining to the waveguidecores is sufficiently low to ensure negligible propagation loss. Thisfirst layer of silica is referred to as the buffer layer. On top of thebuffer layer a layer of doped-silica is deposited (step c), containingone or more dopants that effectively act to increase the refractiveindex of said layer. This doped layer of silica glass is referred to asthe core layer. Depending upon the method used to deposit the core layera high temperature treatment (known as an anneal step) may beadvantageous in order to stabilize the optical and/or mechanicalproperties of said layer. The optical waveguide circuitry is definedthrough standard optical lithography where a UV-transparent platecontaining typically a chromium pattern replica of the waveguide designpattern and possible other structural elements (such as stress relievingand transversal elements) (step d) is pressed against a layer ofUV-sensitive polymer which has been spin coated onto the surface of thecore silica layer, subsequently the UV-sensitive polymer is exposedthrough the mask and the pattern is developed (step e). Following theexposure and development of the waveguide pattern into the polymerlayer, the polymer pattern is used as masking material for dry etching(e.g. RIE—Reactive Ion Etching, ICP—Inductively Coupled Plasma) into thecore silica layer (step e). Alternatively a second masking material issandwiched between the silica core layer and the UV-sensitive polymerlayer, which is used to enhance selectivity and waveguide core profile.In this way the design waveguide pattern is transferred into the coresilica layer having predetermined cross-sectional properties as well asrefractive index. In order to protect the recently defined waveguidecore, and in order to enhance symmetry in the structure transverse tothe direction of propagation a layer of silica with optical propertiesas close to those of the buffer layer as the chosen fabricationtechnology permits is deposited on top of the core structure (step f).The formation of the latter layer (e.g. termed the upper cladding layer)may be formed using successive deposition and annealing steps (step f).

Various relevant aspects of the silica-on-silicon technology is e.g.discussed in M. Kawachi, “Silica waveguide on silicon an theirapplication to integrated-optic components”, Opt. Quant. Electr. 22(1990) 391-416, which is incorporated herein by reference. Variousrelevant aspects of low loss plama enhanced chemical vapour depositedplanar waveguides are e.g. discussed in Christian Laurent-Lund, “PECVDgrown Multiple Core Planare Waveguides with Extremely Low InterfaceReflections and Losses”, Photon. Technol. Lett. 10 (1998) 1431-1433,which is incorporated herein by reference. Various aspects of thetechnique of cladding deposition and reflow annealing usingboron-phosphorus silica glass (BPSG) are disclosed in U.S. Pat. No.6,044,192, which is incorporated herein by reference.

Some preferred embodiments have been shown in the foregoing, but itshould be stressed that the invention is not limited to these, but maybe embodied in other ways within the subject-matter defined in thefollowing claims. The components described above are intended for usewith light propagated from input to output. It is, however, possible toreverse the direction of light propagation, so that light is propagatedfrom an output to an input of a component.

1. An optical branching unit (1) formed on a substrate, the opticalbranching unit comprising waveguides for guiding light at apredetermined wavelength λ, the waveguides comprising a core regionhaving a refractive index n_(core), the core region being embedded in acladding (6) having a refractive index n_(clad), the waveguidescomprising an input waveguide with an input core region (2) of widthw_(in) and at least two output waveguides having output core regions(301, 302) of widths w_(out,i), a branching part (4)—having a refractiveindex n _(core)—for connecting the input and output waveguide cores, asplitting region (7) adjacent to the branching part, the width of thebranching part being substantially equal to w_(in) at its joint with theinput waveguide core and to the sum of the widths w_(out,i) at its jointwith the output waveguide cores, the width of the branching partgradually expanding from its joint with the input waveguide core toallow the output waveguide cores to be branched off and diverge fromeach other in the splitting region wherein a multitude of M transversalwaveguide core elements (5; 501, 502, 503, 504, 505, 506, 507, 508, 509,510) each having a width w_(i), a refractive index n_(trans,i) and beingembedded in said cladding are located in the splitting region formingpaths with a mutual centre to centre distance of s_(i), said transversalwaveguide core elements fully or partially connecting neighbouringoutput waveguide cores.
 2. An optical branching unit according to claim1 wherein opposing edges of neighbouring diverging output waveguidecores meet at the joint with the branching part in a fork or Y-typestructure.
 3. An optical branching unit according to claim 1 whereinsaid branching part comprises a tapered part joining the input andoutput waveguide cores, the width of the tapered part beingsubstantially equal to w_(in) at its joint with the input waveguide coreand to the sum of the widths w_(out,i) at its joint with the outputwaveguide cores, and an abutting region, the output waveguide coreregions being aligned with and extending from said tapered region andabutting each other in the abutting region.
 4. An optical branching unitaccording to claim 1 wherein the optical branching unit has 1 input and2 output waveguides.
 5. An optical branching unit according to claim 1wherein the width w_(i) of the transversal waveguide core elementsdecreases with increasing i as the output waveguide cores diverge.
 6. Anoptical branching unit according to claim 1 wherein the centre to centredistance s_(i) between the i'th and the (i+1)'th transversal waveguidecore element increases with increasing i as the output waveguide coresdiverge or run in parallel.
 7. An optical branching unit according toclaim 1 wherein the transversal waveguide core elements runsubstantially mutually parallel and perpendicular to the outputdirection of the optical branching unit.
 8. An optical branching unitaccording to claim 1, wherein at least one and preferably all of thetransversal waveguide core elements form an uninterrupted path betweentwo neighbouring output waveguide cores.
 9. An optical branching unitaccording to claim 1 wherein the cladding (6) comprises lower (61) andupper (62) cladding layers, the core region (301) of a waveguide beingformed in a layer applied to the lower cladding layer (61) supported bythe substrate (10) and the upper cladding layer (62) being applied tocover the core region (301) and the lower cladding layer (61).
 10. Anoptical branching unit according to claim 9 wherein the upper claddinglayer (62) comprises boron and/or phosphorus doped silica glassdeposited by plasma enhanced chemical vapour deposition as a successionof individually annealed layers.
 11. An optical component comprising acombination of planar waveguides on a substrate, each waveguide having acore region pattern surrounded by lower and upper cladding layers and across-section having a width, the core region pattern being formed in alayer applied to the lower cladding layer supported by the substrate andthe upper cladding layer being applied to cover the core region patternand the lower cladding layer, the combination of waveguides comprisingsubstantially parallel waveguide core sections adjacent to waveguidesections that diverge from said substantially parallel waveguide coresections or the combination of waveguides comprising merging waveguidecore sections comprising a merged core section, where a width of themerged core section and/or a sum of respective widths of the coresections is substantially constant along said waveguide core sections,said optical component further comprising at least one solid voidreducing or stress reducing structural element located in the vicinityof said waveguide core sections.
 12. The optical component as claimed inclaim 11 wherein said structural element includes a stress relievingelement.
 13. The optical component as claimed in claim 12 wherein aminimum distance between a first waveguide and said stress relievingelement is smaller than three times a height of said first waveguide.14. The optical component as claimed in claim 12 wherein said stressrelieving element is elongate and has a width that is less than or equalto a width of a nearest waveguide.
 15. The optical component as claimedin claim 11 wherein said structural element includes a plurality ofparallel running stress relieving elements.
 16. The optical component asclaimed in claim 15 wherein a distance between neighbouring stressrelieving elements is less than 15 μm.
 17. The optical component asclaimed in claim 12 wherein said stress relieving element has widthdimensions that are larger than a nearest waveguide.
 18. The opticalcomponent as claimed in claim 17 wherein said stress relieving elementhas a form that substantially matches the space between two merging ordiverging waveguide core sections.
 19. The optical component as claimedin claim 12 wherein said component is a branching element.
 20. Theoptical component as claimed in claim 12 further comprising transversalelements formed in the waveguide core layer and connecting saidwaveguide core sections.
 21. The optical component as claimed in claim11 wherein said structural element includes segmented sections having anumber of separate waveguide core pieces.
 22. The optical component asclaimed in claim 21 wherein two spaced waveguide sections form part ofan optical coupler with said waveguide core pieces being essentiallyformed as parallelograms when viewed in a planar cross section.
 23. Theoptical component as claimed in claim 21 comprising two spacedsubstantially parallel waveguide sections wherein cross sections of thetwo waveguide sections when viewed in a planar cross section are mirrorsymmetric around an axis midway between the centre axes of the twowaveguide sections.
 24. The optical component as claimed in claim 21wherein spacing between each waveguide segment in a direction ofintended light transmission of a waveguide section is identical for allsegments.
 25. The optical component as claimed in claim 22 wherein anangle of a parallelogram 90°+α defining a waveguide piece as defined byan edge of one waveguide section facing the other waveguide section andthe first edge encountered by light propagated in the intended directionof light transmission is larger than 90°.
 26. The optical component asclaimed in claim 25 wherein the angle α is around 8°.
 27. The opticalcomponent as claimed in claim 21 further comprising transversalwaveguide core elements between segmented waveguide sections.
 28. Theoptical component as claimed in claim 27 wherein the transversalwaveguide core elements of a waveguide section are angled compared to anintended direction of light transmission of the waveguide section. 29.The optical component as claimed in claim 28 wherein the transversalwaveguide elements meet corresponding waveguide segments at an anglesubstantially equal to 90−α.
 30. The optical component as claimed inclaim 27 wherein the transversal waveguide elements are segmented. 31.The optical component as claimed in claim 11 wherein said component is acoupler and the combination of waveguides includes a length of at leasttwo spaced waveguide core sections, said structural element includingtransversal elements arranged between said spaced waveguide coresections, said at least two waveguide sections having, over a certainlength, substantially parallel sections that diverge from each other atboth ends of the parallel sections.
 32. The optical component as claimedin claim 31 wherein two spaced waveguide sections are substantiallyparallel with cross sections of the two waveguide sections andconnecting transversal elements, when viewed in a planar cross section,being mirror symmetric around an axis midway between the centre axes ofthe two waveguide sections.
 33. The optical component as claimed inclaim 32 wherein the transversal elements of a waveguide section areangled compared to an intended direction of light transmission of thewaveguide section to minimize back-reflections.
 34. The opticalcomponent as claimed in claim 33 wherein said spaced waveguide coresections are segmented, each having a number of waveguide core piecesseparated by a space filled with upper cladding material.
 35. A methodof manufacturing an optical component having a combination of planarwaveguides on a substrate, each waveguide having a core region patternsurrounded by lower and upper cladding layers and a cross-section havinga width, the core region pattern being formed in a layer applied to thelower cladding layer supported by the substrate and the upper claddinglayer being applied to cover the core region pattern and the lowercladding layer, the combination of waveguides comprising substantiallyparallel waveguide core sections adjacent to waveguide sections thatdiverge from said substantially parallel waveguide core sections or thecombination of waveguides comprising merging waveguide core sectionscomprising a merged core section, where a width of the merged coresection and/or a sum of respective widths of the core sections issubstantially constant along said waveguide core sections, said opticalcomponent further comprising at least one solid void reducing or stressreducing structural element located in the vicinity of said waveguidecore sections, the method comprising the steps of: a) providing asubstrate; b) forming a lower cladding layer on the substrate; c)forming a core layer on the lower cladding layer; d) providing a coremask comprising a core pattern corresponding to the core region layoutand a layout of said structural elements in the vicinity of saidwaveguide core sections; e) forming core sections and structuralelements using said core mask, a photolithographic and an etchingprocess; and f) forming an upper cladding layer to cover the waveguidecore sections, the structural elements and the lower cladding layer. 36.The method as claimed in claim 35 wherein the step of providing asubstrate includes providing a silicon substrate, and the core andcladding layers include silica glass.
 37. The method as claimed in claim35 wherein the step of forming an upper cladding layer includes formingan upper cladding layer having a lower flow temperature than that of thecore and the lower cladding layer.
 38. The method as claimed in claim 37wherein the upper cladding layer is formed including boron and/orphosphorus.
 39. The method as claimed in claim 35 wherein at least someof the layers on the substrate are formed by plasma enhanced chemicalvapour deposition.
 40. The method as claimed in claim 37 wherein step f)includes successive deposition and annealing steps.
 41. The method asclaimed in claim 35 wherein step e) includes forming said core sectionsand structural elements that include transversal elements that extendbetween at least two of said waveguide core sections so that said atleast two core sections are fully or partially connected by saidtransversal elements.
 42. The method as claimed in claim 41 wherein thewaveguide core sections that are fully or partially connected bytransversal elements are formed to run essentially parallel over acertain length of the waveguides.
 43. The method as claimed in claim 41wherein the waveguide core sections that are fully or partiallyconnected by transversal elements are formed to essentially diverge fromeach other over a certain length of the waveguides.
 44. The method asclaimed in claim 41 wherein at least one of the transversal elements isformed to fully connect two waveguide core sections.
 45. The method asclaimed in claim 35 wherein step e) includes forming said core sectionsand structural elements that include stress relieving elements in thevicinity of said waveguide core sections.
 46. The method as claimed inclaim 45 wherein a flow temperature of the upper cladding layer isadapted so that the waveguide core sections do not flow during anannealing that flows the upper cladding layer.
 47. The optical componentas claimed in claim 19 wherein said branching element is a coupler or asplitter.
 48. The optical component as claimed in claim 12 wherein saidstress relieving element is made of the same material and in the sameprocess step as the core region patterns.
 49. The optical component asclaimed in claim 12 wherein a minimum distance between a first waveguideand said stress relieving element is smaller than twice a height of saidfirst waveguide.
 50. The optical component as claimed in claim 12wherein a minimum distance between a first waveguide and said stressrelieving element is smaller than a height of said first waveguide. 51.The optical component as claimed in claim 15 wherein a distance betweenneighbouring stress relieving elements is less than 10 μm.
 52. Theoptical component as claimed in claim 15 wherein a distance betweenneighbouring stress relieving elements is less than 5 μm.
 53. An opticalcomponent comprising a combination of planar waveguides including inputwaveguides, output waveguides and a transitions section on a substrate,each waveguide having a core region pattern surrounded by lower andupper cladding layers, the core region pattern being formed in a layerapplied to the lower cladding layer supported by the substrate and theupper cladding layer being applied to cover the core region pattern andthe lower cladding layer, said component defining a coordinate system inthe plane of the substrate with origin in the geometric center of saidcomponent, a first axis along an overall direction of light propagationand a second axis perpendicular to the first axis, said componentdefining two outline curves along outermost waveguides measured alongsaid second axis, said outline curves being a monotonically increasingor decreasing function from the origin of the coordinate system and thecombination of waveguides comprising at least one solid void reducing orstress reducing structural element located in a vicinity of saidtransitions section, said input waveguides and/or said outputwaveguides.
 54. The optical component of claim 53, wherein said planarwaveguides comprise segmented waveguide sections comprising one or moregaps so that an outline curve is taken as a curve comprising saidsegments and continuously traversing said gaps with curve segments ofminimal length.
 55. The optical component of claim 53, wherein saidoutline curves are continuous.
 56. The optical component of claim 55,wherein the optical component is a branching unit.
 57. The opticalcomponent of claim 55, wherein the optical component is a splitter orcoupler.
 58. An optical component comprising a combination of planarwaveguides on a substrate comprising at least one branching unitselected from the group of coupler and splitter, each waveguide having acore region pattern surrounded by lower and upper cladding layers and awidth of the transversal cross section, the core region pattern beingformed in a layer applied to the lower cladding layer supported by thesubstrate and the upper cladding layer being applied to cover the coreregion pattern and the lower cladding layer, said optical componentfurther comprising at least one solid void reducing or stress reducingstructural element located in the vicinity of said branching unit.